A system of driverless vehicles driven by frictional contact between a drive tube and a drive wheel on the vehicle is disclosed in U.S. Pat. No. 3,356,040. A vehicle of the type adapted for use herein is taught by U.S. Pat. No. 3,818,837. The present invention is directed to a solution of the problems of how to automatically reduce or increase the speed of a driverless vehicle as it moves along the tracks of a conveyor.